Class Definition Reference
Specifies whether the index should enforce uniqueness.
To specify that InterSystems IRIS should enforce uniqueness for the properties on which this index is based, use the following syntax:
Index name On property_expression_list [ Unique ];
Otherwise, omit this keyword or place the word Not
immediately before the keyword.
The Unique keyword, if present, indicates that this is a unique
The property (or properties) indexed by a unique index are constrained to have unique values (that is, no two instances can have the same collated value) within the extent (set of all objects) of the class (table) that defines the index.
A unique index cannot also be a bitmap
Index SSNIdx On SSN [ Unique ] ;
If you omit this keyword, InterSystems IRIS does not
enforce uniqueness for the properties on which this index is based.